Patients who are immobilized for any number of reasons are prone to develop pressure sores at any point in the body wherein a load bearing bone of the body is adjacent to the underlying support structure. Pressure sores are defined as localized areas of necrotic tissue over bony prominences. Particularly susceptible to pressure sores are patients who are victims of spinal cord injury. Also at risk are stroke, hip fracture and brain injury victims, as well as paraplegics, and geriatrics. The successful cure of pressure sores requires daily care and debridement for a period of many months. In some cases, surgical skin grafting is necessary. The apparent problems and costs of treating a single pressure sore are great and therefore prevention must be considered a prime factor.
It has been estimated, that when seated on a flat surface, up to 75% of a person's body weight is distributed over an area covering only four square inches. This is the area of the ischia or seat bones. For a person weighing 150 pounds, this means as much as 110 pounds pressure is distributed over four square inches, or over 25 pounds per square inch pressure. It is also estimated that the capillary blood pressure, when seated, is about 40 millimeters of mercury or about 0.8 pounds per square inch. If pressures higher than 40 millimeters of mercury are developed during sitting, it is obvious that flow of blood through the capillaries can be restricted, which produces a much higher probability of developing pressure sores or decubitus ulcers.
The prior art has attempted to relieve the occurrence of pressure sores on the human body by supporting the human body on large bodies of liquid, to distribute the supporting force uniformly all over the supported body. These attempts have been successful to a limited extent. However, in order to provide for complete support, it is necessary to displace a weight of liquid equal to the weight of the body being supported. Consequently, such systems are very heavy and are difficult to employ even in beds. Moreover, such heavy liquid cushioning devices have been found difficult to manipulate by patients who utilize wheelchairs to afford them a reasonable degree of mobility and independence, particularly when it is necessary to fold a wheelchair up when it is transferred by automobile.
The prior art patents have disclosed the use of air bags and airtight envelopes which sometimes include a mass of compressible, resilient material to augment the support, see for example U.S. Pat. Nos. 1,332,933, 2,997,100, 3,870,450 and 4,005,236. Other systems have included elastomeric foams which are filled with liquids, see for example U.S. Pat. Nos. 3,611,455, 3,702,484 and 3,965,508. While both of the liquid and gas (air) systems have been successful to some extent in distributing the body load over a wider surface of the body, both suffer from inherent disadvantages. Those fluid filled systems which rely on airtight or watertight envelopes are subject to inadvertent punctures and deflation with the resulting rapid loss of support. The systems based on the use of liquid fillings are invariably heavy and are difficult to use in a wheelchair or other ambulatory devices. Moreover, the systems which rely on air or other gas for support need frequent adjustment to compensate for temperature variations, pressure variations (such as caused by altitude changes) and the weight of the user.
In addition to the foregoing problems, the prior art seat cushions do not provide sitting stability or posture support to the user. Problems with seat stability are particularly critical with respect to wheelchair users. A cushion which provides a stable seating enhances the user's ability to wheel and turn the chair by grasping the wheels, to get in and out of the chair, to reach the floor to pick up an object from the floor and other types of movements. A cushion which provokes non-stability or a fear of falling from the chair will inhibit the user's range of movement. However, a seat cushion which provides a stable and comfortable seat will improve the user's equilibrium and sense of orientation.
Many of the prior art cushions which provide improved pressure distribution, such as those filled with liquid or with other fluids, are not longitudinally divided. Accordingly, when people lean to one side or the other, which is the usual tendency, the fluid moves away from the lower side and into the higher side of the cushion, exaggerating the tendency to lean. This results in the person not only leaning more on a fluid filled cushion than a solid cushion, but it creates a feeling of instability and a fear of leaning to one side because of the instability which is likely to follow.
The prior art seat cushions have also failed to effectively deal with the problem of slumping. The prior art devices have no provision to prevent the ischial tuberosities from gradually sliding forward, which results in slumping to the point where the person's posture becomes bad. The consequences of slumping include increased pressure on the coccyx and, if carried to extreme, a tendency to slip off the cushion and possibly out of the wheelchair.
The seat cushions described and claimed in Applicant's U.S. Pat. Nos. 4,588,229 and 4,726,624 have successfully dealt with most of the foregoing problems, as is evidenced by the commercial success therein. However, an additional problem has been observed when such cushions are used in wheelchairs with reclining backs. Such wheelchairs have provisions allowing the back to be tilted backwardly, thus allowing the patient to recline. This shifts the center of gravity of the patient to the rear in the wheelchair. It has been discovered that in the reclining position, the rearward position of the patient's weight tends to cause the flowable fluid filling material to flow to the forward portion of the pad, which allows for bottoming out of the patient when the patient is returned to the normal seating position. In addition, a patient who slouches excessively can cause the fluid to pool behind his ischial tuberosities, causing a bottoming-out and a resulting pressure sore. Although the flowable fluid may be worked back to the rear of the cushion, damage from the bottoming-out problem can occur before the patient is aware of the problem.
The seat cushion of the present invention overcomes the foregoing problems as is more fully described below.